1. Field of the Invention
The present invention relates to a protective circuit for a lighting system used in a motor vehicle, in particular, it relates to an overvoltage protective circuit for a lighting system using a lamp.
2. Description of the Related Art
A conventional protective circuit for a lighting system used in a motor vehicle is shown in FIG. 4.
A protective circuit shown in the figure, usually, uses a 12 V battery and is composed of a power supply 1, a lamp (light source) 2 for lighting a meter panel, etc. in a motor vehicle, which is energized by the power from the power supply 1, and a driver 3 for driving the light source 2. A MOSFET is used in the driver. The driver controls the power to be supplied to the light source 2 by the power supply 1 according to a control signal (described below).
A voltage detector 4 monitors the voltage of the power supply 1 and when the voltage exceeds 12 V, which is a normal operating voltage level, a signal generated by the occurrence of an overvoltage is supplied to a voltage controller 5. The voltage controller 5 comprises a pulse-width modulated (PWM) pulse generator and lowers the duty ratio of a pulse signal output from a voltage setter 6 in response to a signal received from the voltage detector 4.
The voltage setter 6 is composed of variable resistors and sets the duty ratio of the output of the PWM pulse generator, and when an abnormal voltage is generated, it lowers the duty ratio of the output of the PWM pulse generator according to a signal from the voltage controller 5.
A comparing circuit 7 is a logic circuit composed of a comparator and it receives the output of the voltage setter 6 at an inverting terminal and receives the output of a chopping wave generator 8 at a non-inverting terminal, and compares the levels of both terminals. When the level of the non-inverting terminal is higher than the level of the inverting terminal, the comparing circuit 7 outputs a control signal to the driver 3.
A chopping wave generator 8 is composed of a square wave generator and an integrating circuit. It generates chopping waves utilizing the inclination of a build-up waveform of the integrating circuit in inputting a square wave signal generated by a square wave generator to the integrating circuit.
The operation of the above-mentioned protective circuit will be explained using FIG. 5 in the following.
When the voltage of the power supply 1 is lower than a predetermined voltage, 12 V, the output level of the voltage setter 6 is set to be lower than the peak value, 12 V, of the output level of the chopping wave generator, so that the output waveform of the comparator 7 is a square waveform of a constant width.
When the voltage of the power supply 1 exceeds 12 V, a signal is transmitted to the voltage controller 5 from the voltage detector 4. The peak value of the output waveform of the chopping wave generator becomes also higher than 12 V following the voltage surge of the power supply 1, because the voltage from the power supply 1 is also applied to the chopping wave generator. The voltage controller 5 outputs a control signal to the voltage setter 6 to raise the set value. The set value of the voltage setter 6 is raised based on the control signal, so that, as shown in FIG. 5, the output waveform from the comparator 7 becomes narrower (low duty ratio) as the voltage becomes higher; thereby, the output is controlled to have a constant current flow quantity.
In the case of a conventional protective circuit, however, as a countermeasure for a rush current which occurs at the ON operation of the power supply 1 to energize a lamp, the duty ratio of the PWM circuit is controlled. In this case, there has been a problem as shown below: after an ON operation of the power supply 1, when an overcurrent occurs influenced by the voltage surge, etc. of the power supply 1, since there is no protective function for the overcurrent from the power supply 1, a sufficiently protective function for reducing the overvoltage which is applied to the light source 2 cannot be executed; therefore, when such a overvoltage occurs repeatedly, the lives of parts, especially the life of a lamp, in a lighting system can be shortened.
For the solution of such a problem, a special and high priced IC called an illumination control IC for a motor vehicle, a PWM signal generator IC, has to be used, which raises the cost of the device.